A Spitzer mid-infrared spectral survey of mass-losing carbon stars in the Large Magellanic Cloud

Abstract

We present a Spitzer Space Telescope spectroscopic survey of mass-losing carbon stars (and one oxygen-rich star) in the Large Magellanic Cloud. The spectra cover the wavelength range 5--38 micron. They show varying combinations of dust continuum, dust emission features (SiC, MgS) and molecular absorption bands (C2H2, HCN). A set of four narrow bands, dubbed the Manchester system, is used to define the infrared continuum for dusty carbon stars. The relations between the continuum colours and the strength of the dust and molecular features are studied, and are compared to Galactic stars of similar colours. The circumstellar 7-micron C2H2 band is found to be stronger at lower metallicity, from a comparison of stars in the Galaxy, the LMC and the SMC. This is explained by dredge-up of carbon, causing higher C/O ratios at low metallicity (less O). A possible 10-micron absorption feature seen in our spectra may be due to C3. This band has also been identified with interstellar silicate or silicon-nitrite dust. The line-to-continuum ratio of theSiC and MgS bands shows some indication of being lower at low metallicity. The MgS band is only seen at dust temperatures below 600 K. We discuss the selection of carbon versus oxygen-rich AGB stars using the J-K vs. K-A colours, and show that these colours are relatively insensitive to chemical type. Metal-poor carbon stars form amorphous carbon dust from self-produced carbon. The formation efficiency of oxygen-rich dust depends more strongly on metallicity. We suggest that in lower-metallicity environments, the dust input into the Interstellar Medium by AGB stars is efficient but may be strongly biassed towards carbonaceous dust, as compare to the Galaxy.

Submitted to MNRAS. 22 pages

Cited by in corpus (17)

• Spitzer SAGE survey of the Large Magellanic Cloud II: Evolved Stars and Infrared Color Magnitude Diagrams
• The Magellanic zoo: Mid-infrared Spitzer spectroscopy of evolved stars and circumstellar dust in the Magellanic Clouds
• Molecules and dust production in the Magellanic Clouds
• The Mass-Loss Return From Evolved Stars to the LMC: Empirical Relations for Excess Emission at 8 and 24 μm
• Spitzer spectroscopy of carbon stars in the Small Magellanic Cloud
• A Spitzer IRS Spectral Atlas of Luminous 8 micron Sources in the Large Magellanic Cloud
• The Spitzer-IRS spectrum of SMP LMC 11
• Optical properties of silicon carbide for astrophysical applications I. New laboratory infrared reflectance spectra and optical constants
• Effects of Metallicity on the Chemical Composition of Carbon Stars
• Discovery of Extreme Carbon Stars in the Large Magellanic Cloud
• The trigger of the AGB superwind: the importance of carbon
• The circumbinary disc around the J-type C-star IRAS 18006-3213
• The LMC's Top 250: Classification of the Most Luminous Compact 8 micron Sources in the Large Magellanic Cloud
• Dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
• Mid Infrared Photometry of Mass-Losing AGB Stars
• Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy
• The star formation process in the Magellanic Clouds